The detection of chemical warfare agents in field environments represents a very challenging analytical problem. These toxic agents must be detected at trace concentrations under conditions where other background vapors may be present at much higher concentrations. Therefore, detectors for this purpose must be extremely sensitive, yet highly selective at the same time. Such detectors must provide reliable reports of hazards without generating false alarms.
During the 1980's small chemical sensors based on microelectronic devices emerged as the leading technology for a variety of chemical detection problems. A number of these sensor technologies have been developed and investigated, including optical waveguide chemical sensors, chemiresistors, and surface acoustic wave (SAW) vapor sensors.
For example, U.S. Pat. No. 4,869,874 (Falat) discloses an environmental monitoring apparatus which includes sensors which measure pressure, temperature, relative humidity, and the corrosive nature of the environment. The signals generated by the sensors are stored in a memory module wich may interface with a convention personal computer.
U.S. Pat. No. 4,661,913 discloses an optical comparator and classifier device which operates by having training objects which moved substantially one at a time, in a fluid flow stream. An incident beam of illumination is provided so as to be directed at the training objects in the flow stream. Data associated with each moving training object as the object passes through the beam of illumination is collected. A class of objects is established from this data, these objects having common characteristics based on the data detected from such class of objects. The data is then stored. This stored data is compared to data detected from sample objects of an unknown class. A determination is then made that the objects from the unknown class belong to the established class as a result of directly matching the respective data.
U.S. Pat. No. 5,014,217 (Savage) discloses an apparatus and method for automatically identifying the chemical species within a plasma reactor environment. The apparatus and method include a means for measuring an optical emission spectrum of the chemical species in the plasma and a library containing a multiplicity of predefined spectral patterns. A processor automatically correlates the spectrum with the predefined spectral patterns in the library and yields a correlation value for all the correlations. A subset of the predefined spectral patterns based upon the highest correlation values are selected and used to identify the chemical species and abundances thereof in the plasma.
U.S. Pat. No. 5,120,421 (Glass et al.) discloses an electrochemical sensor which utlizes a multielement, microelectrode array detector for gathering a plurality of signals which form a profile. This profile is then compared to a library of profile responses to determine which library profile best matches the current profile. By determining the closest profile, the compound may be identified.
Both the SAW and chemiresistor chemical microsensors are capable of detecting chemical warfare (CW) agents with high sensitivity. The selectivity provided is substantial but can be improved dramatically through the use of arrays of sensors whose responses are analyzed using pattern recognition algorithms. The above patents disclose the use of a rudimentary pattern recogniton algorithm, i.e. direct correlation to a test set.
SAW sensors are readily adapted to sensor array systems because of the great flexibility in tailoring the chemical selectivity of the individual sensors, and the in depth understanding of their response mechanisms. As a result, SAW vapor sensors have become the leading microsensor technology for chemical warfare agent detection.
The development and testing of sensor systems is an interdisciplinary effort that requires a variety of expertises. These areas include microsensor devices, vapor chemical and physical properties, chemically selective coating materials, microsensor arrays and pattern recognition, sensor system design and integration, automated sensor testing, and finally sensitivity and selectivity concepts.
Other patents of interest in the area of detection systems include U.S. Pat. No. 4,979,124 (Sachse et al.) which discloses a method and system which utilizes an adaptive neural-based signal processor for analyzing emission signals emenating from a test medium for the purpose of determining characteristics of the test medium. In addition, U.S. Pat. No. 5,121,338 (Lodder) discloses a method for using spectral analysis to detect subpopulations. According to this method, a training set of spectra of a first plurality of samples is obtained and a bootstrap distribution is formed therefrom. A test set of spectra from a second plurality of samples is then obtained and a second bootstrap distribution is formed therefrom. First and second univariate distributions are then formed from the respective bootstrap distributions. A quantile-quantile relationship of the training sets is then developed and a determination of whether the test set and the traning set are substantially identical is made from this relationship.